Cardiac medical devices can include diagnostic monitoring equipment, external medical device systems, and implantable medical devices (IMDs). Examples of IMDs include implantable hemodynamic monitors (IHMs), implantable cardioverter-defibrillators (ICDs), cardiac pacemakers, cardiac resynchronization therapy (CRT) pacing devices, and drug delivery devices.
ICDs are generally configured with one or more electrodes for sensing cardiac signals for monitoring the patient's heart rhythm. Upon interpreting one or more of the cardiac signals as arrhythmic, the ICD, in turn, can be used to deliver an appropriate therapy to the patient, with such therapies including pacing and/or defibrillation. Unfortunately, identifying the exact source of an arrhythmia can sometimes be a challenge to both automatic ICDs and the physicians taking care of patients with such ICDs.
For example, distinguishing between ventricular tachycardia (VT) and supraventricular tachycardia (SVT) can pose challenges. VT originates in the ventricular region of the heart, while SVT originates above the ventricular region of the heart, e.g., in the atrial region of the heart or the atrioventricular (AV) node. VT is considered the more severe cardiac event of the two for the patient.
One reason VT and SVT can be difficult to distinguish is that measured cardiac signals, for example an intracardiac electrogram (EGM) or a subcutaneous ECG signal, for each of these types of arrhythmias can be similar, and in turn, interpreted similarly by an ICD. When SVT occurs, the ICD may detect the fast ventricular rate as VT and deliver unneeded therapy to the ventricle. Such unneeded therapies, especially shock therapies, can be physically and emotionally distressful for patients and unnecessarily depletes the ICD battery.
SVT and VT are particularly difficult to distinguish from each other when SVT occurs with 1:1 antegrade conduction or when VT occurs with 1:1 retrograde conduction. In either of these situations, the ventricular and atrial rates will be equal making the two different rhythms appear similar on recorded cardiac signals.
In an attempt to minimize the occurrence of unneeded therapies, various algorithms to more effectively utilize the quantitative aspects of cardiac signals (e.g., EGM morphology, timing intervals between sensed R-waves and P-waves, patterns of sensed R-waves and P-waves, etc). Such evaluations of EGM signals have generally been found to have variable success. By implementing arrhythmia discrimination algorithms within ICDs, the incidence of unneeded therapy can be reduced from that of early generation ICDs. However, unneeded therapies can still occur.
What is needed are medical devices and systematic methods used to prevent the incidence of unneeded therapy, while also being adapted to limit other risks to the patient when using such devices and methods.